1. Field of the Invention
The present invention relates to a field of refresh technique for a pseudo SRAM or a DRAM (Dynamic Random Access Memory) mounted on a mobile device, and more particularly, to a partial array self-refresh type technical field only for a partial area which is set in a memory array in long period refresh operation for reducing power consumption as compared with normal state.
2. Related Art
In recent years, a large-capacity DRAM is mounted in a mobile device such as a cellular phone in general. To reduce the power consumption when the mobile device is in a standby mode, it is strongly desired to reduce the consumption current in a data holding state of the DRAM. To that end, there has been developed a DRAM having long period refresh function for executing self-refresh operation for sufficiently long period as compared with normal operation.
In long period refresh, as an effective technique for further reducing the power consumption, a partial array self-refresh method is proposed (e.g., see Japanese Patent Application Laid-open No. 2004-118938). According to the partial array self-refresh method, in a memory array which generally comprises a plurality of banks, a self-refresh operation is selectively executed for one or some of the banks. In this case, one or some of the banks are set as a holding area, data which must be held are stored in the holding area and in this state, the long period refresh operation may be executed only for the portion corresponding to the holding area. For example, in a partial array self-refresh method in which two of four banks are used as the holding area, since the area to be refreshed is reduced in half as compared with normal refresh operation, and this method is effective for reducing the current consumption of the DRAM.
However, when it is desired to increase the battery using time of the mobile device, the current consumption can not be reduced sufficiently even if the conventional partial array self-refresh method is employed.
Especially as multi-function tendency of the mobile device is increased, the tendency of mounting the large-capacity DRAM is increased, and it is desired to further reduce the current consumption in the self-refresh operation. According to the partial array self-refresh method, if the number of banks in which data is held is reduced, it is possible to reduce the current consumption correspondingly as described above. However, in terms of the ease-of-use of the mobile device, it is necessary to secure the data-holding capacity to a certain extent, and there is a limit of reduction of current consumption of the DRAM in the standby mode.
Moreover, there is a known technique in which power supply is suspended at predetermined timing by controlling predetermined supply voltage which is to be supplied to an array circuit including memory arrays or a peripheral circuit in the self-refresh operation, thereby reducing the current consumption of the DRAM. It is possible to apply this method to the partial array method. In this case, however, of the current flowing in the self-refresh operation, current which can be reduced by suspending the power supply is only supply circuit current (not dynamic operation current, but current corresponding to DC current), and it is necessary to maintain the refresh operation current which is required for the self-refresh operation. Normally, most of current which flows at the time of self-refresh is the refresh operation current, and thus even if the supply circuit current is reduced, sufficient current reducing effect can not be obtained as a whole.